Welcome to SAU Hussain.
You should find all the info your after and more on this site.
To a few of your questions quickly.
Insurance for a turbo compared to a NA R32 will only vary about $50-$300 with most of the insurance companies. When i spoke to Juat Cars about insurance there was only a $80 difference between turbo and NA with no mods.
Its the modifications that you will start to pay for and the excess (eg: extra turbo excess being under 25) and any other modifications to the car.
Its best to ring around and have a chat to a few places to get some ball park figures, that way you will know what you will bo looking at for insrance costs.
Have a read of http://www.skylinesaustralia.com/forums/in...showtopic=12142 it may help to answer some of your questions more.
As for a GTS/t vs a GTS4 it all comes down to preference really and what you want your intentions for the car are? EG: daily driver, weekend cruiser, club/track car (hill climbs, sprint days etc) drags or just a drift pig.
As Paul already mentioned if your thinking of buying a R32 NA then you have to answer yourself a question, WHY???
If its for the looks then there are plenty of other cars that you could buy that will peform just as well mechanically, probably cost less to buy and to run and maintain and insurance wouldn't be as expensive considering your age as well.
No doubt you will ge tbored with it withing a week and want to buy a turbo R32 anyway so if your dead set keen then i would just start off with a stock turbo and go from there (modifications can always be done and all in good time )
There are plenty of cars around withing Australia so you have to decide if you want to go through a dealer/importer or buy one privatley.
Have a read of http://www.skylinesaustralia.com/forums/in...showtopic=35129 if you do decide to buy privately.
You are going to pay about 12K for a nice clean R32 no matter what and dont be fooled into thinking you will saved thousands by going to melbourne to buy one. YES you will save money but if you do your research and dont rush into buying the first car you fall in love with you will pick up a bargain.
Like http://www.skylinesaustralia.com/forums/in...showtopic=94598
if you need a hand at all (if your from south) then give me a holla as im only to hapy to help you out. I have a car you may also be interested in as well so let me know if you want to come check it out and i can take you for a ride.
Cheers B